Polyethylene is the most widely used commercial polymer. It can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE).
A normal set-up of a plant for polymerizing or copolymerizing ethylenically unsaturated monomers such as ethylene and esters of vinyl alcohol in the presence of free-radical polymerization initiators consists essentially of a set of two compressors, a low-pressure and a high-pressure compressor, a polymerization reactor, which can be an autoclave or a tubular reactor or a combination of such reactors, and two separators for separating the monomer-polymer mixture leaving the reactor, wherein in the first separator, the high-pressure separator, the ethylene and comonomers separated from the monomer-polymer mixture are recycled to the reaction mixture between the low-pressure compressor and the high-pressure compressor, and the ethylene and comonomers separated from the mixture in the second separator, the low-pressure separator, are fed to the low-pressure compressor where it is compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the combined streams are further pressurized to the pressure of the high-pressure gas recycle stream. Such a high-pressure polymerization unit normally further includes apparatuses like extruders and granulators for pelletizing the obtained polymer. In case of tubular reactors, monomer supply to the reactor can either be carried out solely in the beginning of the reactor or only partly in the beginning with the other part fed via one or more side feed entries. Moreover, it is also common to introduce initiator in more than one place down the tube, thus creating more than one reaction zone.
The properties and the structure of the obtained ethylene polymers or copolymers, such as molecular weight, molecular weight distribution and the amount of short- and long-chain branching, depend strongly on the reaction parameters pressure and temperature. That means control of the reaction conditions is essential. There are of course additional possibilities to influence the nature of the produced polymers, for example the addition of chain-transfer agents, which reduce the molecular weight, however in general the possibilities to vary the reaction conditions while obtaining a specific target product are quite limited. A further limiting factor for the production of ethylene polymers or copolymers is heat removal from the reactor, because the heat of polymerization of ethylene is relatively high. That means, for obtaining different grades of polymers, i.e., polymers which differ in melt flow rate (MFR) and density, it is in general necessary to adjust the operating parameters, which might result in different output rates.
Radically initiated high-pressure polymerization is an appropriated method for producing copolymers of ethylene and esters of vinyl alcohol, especially for preparing copolymers of ethylene and esters of vinyl alcohol having a vinyl ester content in the range of from 1% of weight to 45% of weight. Suitable reactors can be stirred autoclave reactors or tubular reactors. Such high-pressure polymerizations are for example described in U.S. Pat. No. 4,091,200, EP 012 368 A1 and WO 99/014251 A1. An obstacle for producing copolymers of ethylene and vinyl esters in high-pressure polymerization is however that the comonomers and the produced ethylene-vinyl ester copolymers can degrade to acetic acid or other hydrocarbon acids which are highly corrosive for the polymerization apparatuses.
Moreover, as in nearly all commercial processes, there is a permanent demand for improving the efficiency of the production process. Accordingly, there is a continuous desire to increase the capacity of existing polymerization plants by increasing the production rate of the polymerization reactors without adversely affecting the properties of the produced ethylene copolymers.
Thus, it was the object of the present invention to overcome the disadvantages of prior art processes for the preparation of copolymers of ethylene and esters of vinyl alcohol in the presence of free-radical polymerization initiators and to reduce of the corrosion caused by degradation products of the comonomers and the obtained ethylene-vinyl ester copolymers and to provide a possibility for increasing the production rate in high-pressure copolymerization of ethylene and vinyl esters.